Organic electroluminescent (EL) devices are electronic devices that emit light in response to an applied potential. The structure of an EL device comprises, in sequence, an anode, an organic EL medium, and a cathode. Although organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device includes an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, are usually composed of a polycyclic aromatic hydrocarbon, that are very thick (much greater than 1 μm) and highly resistive. Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element including a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). Still further, there are other multi-layer EL devices that contain additional functional layers, such as an electron-blocking layer (EBL), and/or a hole-blocking layer (HBL) in the devices. At the same time, numerous types of organic materials have been discovered and used in organic EL devices. These new structures and new materials have further resulted in improved device performance.
Besides the above organic EL devices including low molecular weight materials, an EL device wherein the EL element including a polymer such as poly(fluorene) derivative, poly(p-phenylenevinylene) derivative, and poly(thiophene) derivative, and a device including a mixture of a polymer such as poly(vinyl carbazole) with a low molecular weight light emitting material and an electron-transporting material have been developed.
Improving operational stability and lowering the required driving voltage is particularly important for the EL devices to be used in flat panel display. The inherent lack durability of organic materials used in the EL element of an EL device results in crystallization due to the heat evolved from the prolonged passage of current, and therefore shortened device lifetime. During device operation, if the temperature inside of a device (defined as device temperature) is higher than a glass transition temperature (Tg) of an organic layer in an EL device, the organic layer will change its film formation from an amorphous state to a polycrystalline formation. This change will not only cause a film morphology change, but also cause a possible change in its ionization potential (IP) or its electron energy band gap (Eg). As a result, electrical shorts can occur, carrier injection can deteriorate, or luminance efficiency can be reduced.
In particular, much effort has been directed to the discovery of useful electron-transporting materials. There are problems associated with many existing and commonly used electron-transporting materials such as undesired emitting color from electron-transporting material itself, high driving voltage, and poor device lifetime.
Tris(8-hydroxyquinoline)aluminum (Alq), one of the metal chelated oxinoid compounds, has been a commonly used electron-transporting material in OLEDs since Tang et al. disclosed its use in “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987). Alq has a reasonably high Tg (about 172° C.). This property facilitates the operational stability of the EL device at a device temperature up to its Tg. However, the electron mobility of Alq is not as effective as expected and so the driving voltage is high. In addition, there is undesired light emission from Alq when this material is located in an ETL in a blue OLED.
U.S. Pat. No. 5,393,614 disclose a specific phenanthroline derivative, 4,7-diphenyl-1,10-phenanthroline (Bphen), as an electron-transporting material. Due to its high electron mobility and suitable energy band structure, Bphen can efficiently transport electrons from the cathode into the LEL resulting in high luminous efficiency and low drive voltage. However, Bphen has a low Tg (about 60° C.), and a vacuum deposited amorphous Bphen layer in an EL device can be readily changed into a polycrystalline layer during operation, which results in a sudden drop in luminance and a sudden increase in drive voltage. Its operational lifetime is no longer than 20 hrs if the device is operated at 70° C., substantially minimizing the effectiveness of this material in an EL device